Polyurethane foamed material is formed by the chemical reaction of polyol, isocyanate and optionally additives such as stabilizers, activators and colorants. So that the liquid components can react with one another, they must be homogeneously mixed with one another in a suitable way. Various principles are known for carrying out this basic process technology operation. One possible way of mixing the polyurethane raw materials is to use continuously operating stirrer mixers. These stirrer mixers generally have a rotationally symmetrical mixing chamber in which a stirring means (hereinafter also termed stirrer) is rotatably driven by a motor. Optionally, the mixing chamber may include stationary internal fittings that assist the stirring procedure, which are termed counter-pins. The mixing chamber often has a throttle a short way upstream of the outlet opening, by means of which the pressure in the mixing chamber can be regulated during the production procedure. In continuously operating stirrer mixers, the components to be mixed are added radially from outside. The addition to the mixing chamber is effected by screwed-in nozzles or through tubes. The polyurethane reaction mixture that is formed by the mixing of the polyurethane raw materials leaves the mixing chamber through a centrally arranged outlet opening. The components to be mixed are added to the mixer with the aid of pumps.
The stirrers hitherto used in the production of polyurethane foamed material, such as the commonly employed pin stirrer, only perform the task of homogeneously mixing the components to produce a polyurethane reaction mixture. In this connection, the flow that is generated by the pumps superimposes a further flow that is generated by the rotation of the stirrer. This stirrer flow generated by the stirrer homogeneously mixes all components with one another. This stirrer flow can form inter alia flow vortices that exert resistance to the flow generated by the pumps. With increasing stirrer speed, the flow resistance of the stirrer mixer thus rises, which is manifested in a higher pressure loss and thus in a higher mixing chamber pressure.
The pumps necessarily cause the reaction mixture to flow through the mixer. The mean residence time of the mixture in the mixer is determined by the throughput of components and the volume of the mixing chamber. The mean residence time is calculated from the quotient of the sum of the volume flows and the mixing chamber volume. The value of the mean residence time is normally in a range from 0.1 to 2.5 seconds. The mixer exerts a hydrodynamic resistance on the flow. Accordingly, a pressure loss is produced that directly determines the pressure in the mixing chamber, the so-called mixing chamber pressure. The mixing chamber pressure is typically measured in the feed line of the polymer stream.
Important process parameters that influence the subsequent quality of the polyurethane foamed material are the stirrer rotational speed, the mixing chamber pressure and the throughput of polyurethane reaction mixture. In the production of polyurethane foamed material, it is known that the mixing chamber pressure has a decisive influence on the number of cells of the finished foamed material (EP-A1-0 565 974). In general, fine-cell foamed material can be produced only at low mixing chamber pressures. At high mixing chamber pressures, coarse-cell foamed material is produced.
In the known stirrers the process parameters, stirrer rotational speed, throughput and mixing chamber pressure cannot be adjusted independently of one another. If the throughput is raised, the mixing chamber pressure also rises. Similarly, the mixing chamber pressure rises with an increase in the stirrer rotational speed.
Apart from this, with an increase in the throughput amount, the mean residence time in the mixing chamber drops. To be able to produce a homogeneously mixed polyurethane reaction mixture under a reduced residence time, a correspondingly higher stirrer rotational speed is necessary. On raising the throughput, the mixing chamber pressure thus rises not only on account of the greater flow resistance due to the increased discharge output, but also due to the necessary rise in the stirrer rotational speed. At high discharge outputs, it is therefore not possible with a given structural size of the stirrer mixer to adjust a low mixing chamber pressure, which is required for the production of fine-cell foamed material.
Stirrer devices are known in the art that effect an axial conveyance, such as inclined blade stirrers and propeller stirrers. These, however, operate in large vessels where the distance to the outer wall is relatively large. For the continuous mixing of polyurethane components with the aim of producing a homogeneous polyurethane reaction mixture, hitherto only stirrer devices that do not effect an axial conveyance have been used. The important feature of stirrer mixers for mixing polyurethane components is above all the short distance remaining between the mixer chamber wall and the rotating stirrer device. Usually, a gap that is not larger than 5 mm remains. Furthermore, the low residence time of normally between 0.1 and 2.5 seconds in the stirrer mixer in the production of polyurethane foamed material is a characteristic feature.